It turns out, Leap Day is more complicated than you realized.
Since the Earth doesn’t take exactly 365 days to revolve around the sun, an extra day is added to the month of February every four years. It turns out, however, the math behind Leap Day is actually much more complicated.
In order to account for the mathematical issue, Leap Day is skipped on “centurial years not divisible by 400.” So, the year 1900 wasn’t a leap, but 2000 was.
The basic concept behind Leap Day is this: It takes the Earth 365 and one-quarter of a day to revolve around the sun. In order to keep the calendar and seasons in sync, Leap Day was invented. Every four years, that extra quarter of a day is accounted for.
Unfortunately, even those numbers are still incorrect.
According to History.com, the Catholic church noticed that Easter had started to drift away from the first Sunday after the first full moon following the Spring Equinox. This was due to the fact that the Earth takes slightly under 365.25 days to revolve around the sun.
Therefore, Leap Day was adding about 11 minutes per year to the calendar.
In order to account for this, Leap Day is skipped on “centurial years not divisible by 400.” So, the year 1900 wasn’t a leap, but 2000 was.
While this mostly fixed the problem, it’s still not perfect. This version of the calendar actually adds an extra day every 3,030 years, so things are going to get a little confusing in the 46th century.